1. Field of Invention
The present invention relates to a method of determining the free-field thrust of a gas turbine engine using an enclosed gas turbine engine test facility. In particular, the present invention concerns a method for calibrating an enclosed gas turbine engine test facility to determine the free-field thrust of such an engine. Furthermore, the presents invention concerns apparatus used for calibration of an enclosed gas turbine engine test facility.
2. Description of Related Art
It is sometimes necessary to accurately measure the thrust produced by a gas turbine engine, for example during certification of an engine type or during pass-off of an individual engine. The thrust of any gas turbine engine will vary according to the ambient conditions in which it operates. It is therefore necessary to standardise the thrust produced by a gas turbine engine to a static or ‘free field’ thrust, produced on an International Standard Day. That is to say, the thrust that would be generated if the engine were operating in an undisturbed atmosphere at precisely defined temperature, pressure and density.
A convenient method of measuring the thrust is to use an enclosed gas turbine engine test facility. Typically, such a facility comprises an enclosure housings a thrust-measuring cradle to which the gas turbine engine is mounted. Inlets in the enclosure allow a substantially undisturbed flow of air into the engine while an exhaust outlet, also known as a detuner or augmenter, provides exit means for the hot exhaust gasses produced.
The enclosed test facility offers a number of benefits. Because the engine is shielded from the elements, testing can take place in consistent conditions, regardless of weather conditions. Also, careful design minimises the environmental impact of engine testing, particularly noise. However, an enclosed test facility suffers limitations. The flow of exhaust gas into the exhaust outlet, often by design, generates an airflow through the test facility. This flow, as much as three times that through the engine, protects the detuner from the exhaust gas but also generates drag. In effect a negative thrust is created that reduces the thrust of the engine by as much as ten percent. Thus the test facility must be calibrated to indicate the thrust that the engine would produce if surrounded by a static atmosphere.
It is common practice to calibrate an enclosed test facility by “back-to-back” tests against an open-air test facility. An open-air test facility comprises a thrust cradle to which an engine is mounted and operated, supported far enough away from external influences, such as the ground, that air around the engine remains substantially unaffected during operation. In this way, the “free field” thrust of the engine can be measured. However, the procedure is costly and time consuming as the external test facility is not as flexible as the enclosed test facility. The open-air facility is, by necessity, exposed to the elements, limiting availability. Furthermore it has a greater environmental impact and may be subject to restrictions on operating times.